(a) Field of the Invention:
This invention relates to a focal plane shutter wherein a plurality of opaque substantially square thin shutter blades are supported by a pair of arms to make an exposure by running them just before a film.
(B) Description of the Prior Art:
In this type of focal plane shutter, the fulcra of the pair of arms are provided on the side of an exposure aperture and the shutter blades are run in the vertical direction just before the aperture. Such various focal plane shutters are already suggested and practiced. The methods of supporting the shutter blades with the arms can be divided into the following two types. The first type is a long arm type. In this type, for example, as disclosed in U.S. Pat. No. 3,871,012, shutter blades are supported by arms on the other side over an exposure aperture than the side on which the fulcra of the arms are provided. Therefore, the arms are longer than the lateral width of the aperture. Then, as the rotating angle of the arm is comparatively small, the displacement in the lateral direction of the shutter blade is also small but, on the other hand, there has been a defect that, if the number of the shutter blades supported by a pair of arms is to be made more than two, the structure will be complicated. Therefore, in order to eliminate this defect, a short arm type has been developed as the second type. In this type, as described later, contrary to the above mentioned long arm type, shutter blades are to be supported on the same side of the aperture, that is, in the positions near the fulcura of the arms and many shutter blades can be very simply supported by a pair of arms but, on the other hand, there has been a defect that the size in the lateral direction is large.
It shall be described in detail in the following with reference to FIGS. 1 and 2. In the drawings, only one set of two sets of blade groups is shown but the other set is substantially identical except that it is arranged symmetrically with respect to the aperture and shall be therefore omitted. In FIGS. 1 and 2, reference numeral 1 indicates a base plate provided with an exposure aperture 1a, numeral 2 indicates a main arm pivoted on the base plate 1 on one side of the aperture 1a by a shaft 3, numeral 4 indicates an auxiliary upper arm pivoted on the base plate 1 on one side of the aperture 1a by a shaft 5. Symbols A, B and C indicate respectively shutter blades supported on the main arm 2 and auxiliary arm 4 respectively by shafts 2a and 4a; 2b and 4b, 2c and 4c. Further, in FIG. 1, symbol d.sub.1 indicates a distance between the shafts 3, 5 and the left edge of the exposure aperture 1a, symbol d.sub.2 indicates a distance between the shaft 5 and the left end edge Aa of the shutter blade A and symbols R.sub.1, R.sub.2 and R.sub.3 indicate respectively distances from the shaft 5 to the shafts 4a, 4b and 4c. By the way, FIG. 1 shows the blade group as unfolded and FIG. 2 shows the blade group as folded. In assembling them, the shutter blade A is first fitted to the main arm 2 and auxiliary arm 4 and then, below it, the shutter blades B and C are fitted onto the arms 2 and 4 so as to be overlapped in turn. In such case, the overlapping order can not be changed. In the conventional short arm type focal plane shutter by such arrangement as in the above, the respective distances from the shaft 5 to the shafts 4c, 4b and 4a are naturally determined by the width in the vertical direction of the aperture 1a, that is, by the moving ranges in the vertical direction of the shutter blades A, B and C. That is to say, the distance R.sub.2 between the shafts 5 and 4b must be set properly by considering the distance d.sub.2 from the shaft 5 to the left end edge Aa of the shutter blade A when such shutter blade group as is shown in FIG. 1 is unfolded, the distance d.sub.2 ' from the shaft 5 to the left end edge Aa of the shutter blade A when such shutter blade group as is shown in FIG. 2 is folded, the thickness of the shaft 4b and the overlapped area of the left end part of the shutter blade A and the base plate 1. Thus, according to the conventional short arm type focal plane shutter, the distance d.sub. 1 from the position of the supporting shaft of the arm to the left edge of the aperture 1a is regulated by the position of the supporting shaft 4b of the shutter blade B, that is, the other shutter blade arranged adjacently to the shutter blade A for forming an exposure slit and therefore can not be made small. Therefore, not only there is a fixed limitation to reducing the dimension in the lateral direction of the focal plane shutter but also, if the number of the shutter blades becomes larger, the distance R.sub.2 will also become longer in response to it, therefore the distance d.sub.1 also can not help being made larger and the dimension in the lateral direction of the shutter has had to become larger. In this kind of focal plane shutter, it can be very easy to increase the number of shutter blades of one set of a blade group, thereby the width of one shutter blade can be made that much smaller and the occupied area when the blade group is folded as shown in FIG. 2 may be so small that the dimension of the height of the shutter can be made small. However, there has been a defect that, when the dimension of the height of the shutter is made small, the dimension in the lateral direction will become large.